Packaging laminates comprising united layers of plastic and/or other materials are often employed for packages of the single-use disposable type, and there occur on the market not only simple, flexible packaging laminates comprising a limited number of layers, but also complex, rigid packaging laminates comprising a plurality of layers in expediently selected mutual combinations.
Irrespective of whether the packaging material is of one type or the other, it is important that the composition of the individual packaging laminate be such that a package produced from the packaging laminate is given the best possible operative preconditions to be capable of storing a product in a manner which reliably safeguards the product. Thus, the requirements on the packaging laminate are that it must possess not only mechanical and chemical but also other desirable—and necessary—properties in order to make for adequate product protection, at the same time as the packaging laminate must, naturally, be economically viable and easy to produce and form into packages on a rational industrial scale.
One conventional packaging laminate for configurationally stable single-use disposable packages comprises a core layer of paper or paperboard which imparts to the package the requisite mechanical strength and configurational stability, and outer coatings of thermoplastic, preferably polyethylene, which impart to the package tightness properties in respect of moisture and liquid. The outer thermoplastic coatings also possess the advantage that they are heat-sealable (or fusible) in such a way that mutually facing thermoplastic surfaces may be brought to surface fusion and bonding with one another under the application of heat and pressure, for the formation of mechanically strong and liquid-tight sealing seams or joints during the reforming of the packaging laminate into packages.
Another conventional packaging laminate for the above-described type of single-use packages also comprises a core layer of paper or paperboard and outer coatings of thermoplastic, preferably polyethylene, and moreover one or more additional layers of material which are intended to impart to the package supplementary tightness properties. One such packaging laminate calls for the employment of aluminium foil (Alifoil) on the one side of the core layer, between the core layer and the outer thermoplastic layer of the packaging laminate, in order to make the package oxygen gas tight when it is to be employed together with products which are sensitive to oxygen gas. Another packaging laminate calls for the use of one or more polymer layers instead of, or as a complement to the above Alifoil, in order to impart to the package oxygen gas tightness and/or tightness properties vis-à-vis certain types of flavorings, essential oils or the like which may occur in the product. Examples of such barrier polymers as have hitherto been employed in the art are polyesters, polyamides, polyvinyl pyrolidone, ethyl vinyl alcohol copolymer (EVOH) etc. Yet a further example of a barrier material might be silicon oxide (SiOx) which also imparts to the package tightness properties in respect of oxygen gas and substances of the above-mentioned type.
Simple, flexible packaging laminates for packages of the bag type may, for instance, include two mutually united plastic layers and an interjacent layer of an Alifoil or barrier polymer of the above-mentioned type.
Both flexible and rigid packaging laminates are normally produced by a lamination process which is based on the concept that prefabricated webs of plastic or plastic-coated other materials are brought together and permanently united with one another by the application of heat and pressure. In, for example, a conventional packaging laminate, a prefabricated web of plastic-coated paper is brought together with a prefabricated film web of barrier polymer and these are led together through the nip between two rotating press rollers, at the same time as a binder or sealing plastic acting as a binder is applied between the webs in order permanently to unite the webs to one another. Alternatively, one and/or the other web may be pretreated with a so-called primer which is applied on the contact surface of the web or webs, respectively, and which is activated by the application of heat during the lamination process, in order to increase, or make possible, adhesion between the webs. In the production of a flexible packaging laminate comprising but two plastic layers, the corresponding procedure is utilized in that a first prefabricated web of plastic and a second prefabricated web of plastic are brought together and led together through the nip between two rotating press rollers, at the same time as a binder or sealing plastic acting as a binder is applied between the webs in order permanently to unite them to one another. Also in this case, one web and/or the other may be pretreated with an adhesion-promoting agent (primer) which is activated by the application of heat during the lamination process.
At the same time as the above-described, conventional film lamination methods produce packaging laminates possessing a high degree of integrity and good internal cohesion between the material layers included, they all suffer from serious drawbacks and inconveniences, in that they employ binding agents (adhesive, primer) which are often associated with environmentally hazardous organic solvents. Even though there are binders and adhesives which do not employ such solvents, it has hitherto proved difficult to incorporate them successfully into the rational, industrial production of packaging laminates with the desired integrity and cohesion and, in certain cases, it has moreover proved unavoidable to employ solvent-based binders or adhesives for permanently uniting films or webs of plastics which defy ready fusion or plastics which cannot be united with one another by surface fusion under the application of heat and pressure (heat sealing).